Optical pick-up system with servo using reflection plate having aperture

ABSTRACT

Optical pick-up system and method for accurately reading information recorded on an optical disc. The invention makes it possible to determine the kind of an error occurring upon reading information recorded on an optical disc by light beams from a light source, by using error detectors, and to correct accurately the error by driving a focusing lens, thereby capable of rapidly correcting errors such as tracking/focusing errors, providing a light and compact construction and improving a reliance, so as to solve problems of a high sensitivity to variations, a poor assembling ability, a bulky construction and a degraded reliance, encountered in optical devices which have been generally used for reading information recorded on optical discs.

FIELD OF THE INVENTION

The present invention relates to an optical pick-up system and method,and more particularly to an optical pick-up system and method capable ofaccurately reading information recorded on an optical disc.

BACKGROUND OF THE INVENTION

Recently, the use of optical discs such as compact discs and laser discshave been greatly increased. In the future, these optical discs will beused in all technical fields, by virtue of their advantages of acapability of storing a lot of information and convenience of use.

However, optical discs require an optical pick-up device for readinginformation recorded thereon.

Moreover, optical devices which have been generally used for readinginformation recorded on optical discs are very sensitive to vibration.As a result, it is very difficult to read accurately informationrecorded on optical discs at places where severe vibrations occur.

Referring to FIG. 1, there is illustrated a general optical pick-updevice. As shown in FIG. 1, the optical pick-up device comprises asemiconductor laser diode 21 as a light source, a half mirror 22positioned downstream of the laser diode 21 and adapted to reflect lightbeams emitted from the laser diode 21 at a predetermined angle, and a45° mirror 23 positioned downstream of the half mirror 22 and adapted toreflect the light beams reflected from the half mirror 22 at an angle of45°. Positioned downstream of the 45° mirror 23 is a focusing lens 24which feeds the light beams reflected from the 45° mirror 23 to anoptical disc 25 so that information recorded on the optical disc 25 canbe read. The optical pick-up device also comprises a condensing lens 26positioned at a path along which light beams reflected from the opticaldisc 25 is reflected. The condensing lens 26 is adapted to condense thelight beams reflected from the optical disc 25. Downstream of thecondensing lens 26, a photodetector 27 is also positioned, so as tosense light beams condensed by the condensing lens 26.

Operation of the optical pick-up device with the above-mentionedarrangement will now be described.

First, light beams emitted from the laser diode 21 is reflected by thehalf mirror 22 so that they are fed to the 45° mirror 23. The lightbeams fed to the 45° mirror 23 are then reflected by the 45° mirror 23at an angle of 45°, so as to be fed to the focusing lens 24. Thefocusing lens 24 focuses the light beams incident thereon and thentransmits them to the optical disc 25. The light beams incident on theoptical disc 25 are then reflected with different reflection amounts,according to forms of information recorded on the optical disc 25.

The light beams reflected from the optical disc 25 are fed to thefocusing lens 24 which, in turn, converts them into parallel lightbeams. The parallel light beams are then fed to the condensing lens 26,so as to be condensed. The condensed light beams are detected by thephotodetector 27. The light beams reflected by the optical disc 21 havedifferent intensities, according to information recorded on the opticaldisc 21. By virtue of such a difference in light amount, the recordedinformation can be read.

However, such a general optical pick-up device has a bulky opticalsystem, since elements thereof should be arranged at individualpositions. As a result, there is a difficulty in applying such anoptical pick-up device to appliances which are on a compacting trend.Moreover, the assembling of elements at their individual positionsresults in a lengthened adjusting time and a poor ability in assembling.Consequently, the manufacture cost and the poor product rate areincreased.

SUMMARY OF THE INVENTION

Therefore, the present invention has been made in view of theabove-mentioned problems encountered in the prior art and an object ofthe invention is to provide an optical pick-up system and method capableof reading accurately information recorded on an optical disc.

Another object of the invention is to provide an optical pick-up systemand method capable of reading accurately information recorded on anoptical disc, even when the optical disc is subjected to vibrations.

Another object of the invention is to provide an optical pick-up systemand method capable of reading accurately information recorded on anoptical disc, by sensing an error caused by vibrations applied to theoptical disc and correcting it.

Another object of the invention is to provide an optical pick-up systemcapable of improving an ability in assembling and having a compact andlight construction, as compared with general ones.

In accordance with one aspect, the present invention provides an opticalpick-up system for reading information recorded on an optical disc, saidsystem comprising: a light source; information reading means for readingsaid information recorded on said optical disc, by using light beamsemitted from said light source; error correcting means for detecting anerror occurring when said information reading means reads theinformation and correcting said error; and supporting means forproviding optical paths for the information reading means and said errorcorrecting means and supporting both the information reading means andthe error correcting means.

In accordance with another aspect, the present invention provides anoptical pick-up method for reading information recorded on an opticaldisc, said method comprising the steps of: reading said informationrecorded on said optical disc by using light beams emitted from a lightsource; detecting an error occurring upon reading the information; andcorrecting said detected error.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and aspects of the invention will become apparent from thefollowing description of embodiments with reference to the accompanyingdrawings in which:

FIG. 1 is a schematic perspective view of a general optical pick-updevice;

FIG. 2 is a schematic perspective view of an optical pick-up systemaccording to the present invention;

FIG. 3 is a schematic sectional view of the optical pick-up systemaccording to the present invention;

FIG. 4A is a schematic sectional view of a two-component photodetectoraccording to the present invention;

FIG. 4B is a schematic sectional view of a three-component photodetectoraccording to the present invention;

FIG. 5 is a schematic sectional view of a reflection plate according tothe present invention;

FIG. 6 is an enlarged view of a portion A of FIG. 3;

FIG. 7 is a schematic view for explaining a design for a hologramaccording to the present invention;

FIG. 8 is a schematic sectional view illustrating optical paths when noerror such as a tracking error or a focusing error has occurred;

FIG. 9 is a partial enlarged view of FIG. 8;

FIGS. 10A to 10D are schematic views for explaining focusing errors;

FIGS. 11A to 11C are schematic views for explaining tracking errors;

FIGS. 12 and 13 are schematic sectional views illustrating optical pathswhen a tracking error has occurred;

FIG. 14 is a schematic sectional view illustrating optical paths formedat conditions of FIGS. 10A and 10C; and

FIG. 15 is a schematic sectional view illustrating optical paths formedat conditions of FIGS. 10B and 10D.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIGS. 2 and 3, there is illustrated an optical pick-upsystem in accordance with an embodiment of the present invention.

As shown in FIGS. 2 and 3, the optical pick-up system comprises a laserdiode 10 as a light source, a beam splitter 20 for transmitting a partof light beams emitted from the laser diode 10 therethrough andreflecting the remaining light beams therefrom, and a prism 30 forreceiving the light beams transmitted through the beam splitter 20.Attached to the prism 30 is a first mirror 70 which is adapted toreflect the light beams incident on the prism 30 at a predeterminedangle. The optical pick-up system also comprises a second mirror 80 forreflecting the light beams reflected by the first mirror 70 at apredetermined angle and a collimating hologram 40 for converting thelight beams reflected by the second mirror 80 into parallel light beams.Positioned downstream of the second mirror 80 is a focusing lens 50which is adapted to focus the parallel light beams emerging from thecollimating hologram 40 and feed them to an optical disc 60. An actuator12 is also provided for shifting the focusing lens 50. Of course, theprovision of the actuator 12 may be omitted in some cases.

Attached to the prism 30 are a three-component photodetector 11, areflection plate 15 and a two-component photodetector 90, all of whichconstitute means for sensing light beams reflected from the optical disc60 and determining whether a tracking error or a focusing error hasoccurred.

The reflection plate 15 is attached to a front surface of the prism 30and is of a disc provided with an opening having a diameter l, as shownin FIG. 5.

As shown in FIG. 4A, the two-component photodetector 90 comprises twophotodetector components D₁ and D₂ divided from each other withreference to a x-axis corresponding to a x-axis (FIG. 3) on which apoint P₂ is positioned. The two-component photodetector 90 also has acentral opening shaped into an ellipse having a z-axis (major axis)length l₁ identical to a width L₈ of FIG. 6 (l₁ =L₈) and a x-axis (minoraxis) length l₂ identical to the diameter l of the opening of reflectionplate 15 (l₂ =l).

As shown in FIG. 4B, the three-component photodetector 11 comprisesthree photodetector components D₃ to D₅, for example, threephotodetectors. Sizes of the photodetector components are determined asfollows.

That is, the sizes of photodetector components D₃ and D₅ are determinedby taking into consideration magnitudes of errors such as a trackingerror and a focusing error. The size of photodetector component D₄ isdetermined by taking into consideration a focusing point formed when noerror has occurred.

The collimating hologram 40 comprises a holographic plate HP which isdesigned to receive diverging spherical waves W₁ emitted at a placespaced a distance L apart from the holographic plate HP and at an angleθ₃ ' with respect to a line normal to the holographic plate HP andparallel waves W₂ emitted at an angle θ₄ ' with respect to the linenormal to the holographic plate HP, as shown in FIGS. 3 and 7. Thedesign of the holographic plate HP is made so that its parameterssatisfy the following conditions:

    θ.sub.3 '<45°,

    θ.sub.3 '=θ.sub.3,

    θ.sub.4 '=θ.sub.4, and

    L=L.sub.1 +L.sub.2 +L.sub.3 +L.sub.4

Also, other parameters shown in FIGS. 3 and 6 should satisfy thefollowing conditions: ##EQU1##

Operation of the optical pick-up system with the above-mentionedarrangement will now be described.

Light beams emitted from the laser diode 10 are fed, via the beamsplitter 20, to the wave guide prism 30 at an incident angle θ₂, asshown in FIGS. 3 and 6. In the prism 30, the incident light beams arerefracted at a refraction angle θ₁ expressed by the following equation,by virtue of the difference in refractive index between the prism 30 andsurrounding air. ##EQU2##

The beams refracted at the angle θ₁ are then fed to the first mirror 70which, in turn, reflects the beams toward the second mirror 80. By thesecond mirror 80, the beams are then fed to the collimating hologram 40at an incident angle θ₃. The light beams incident on the collimatinghologram 40 are reproduced in the form of parallel light beams with adiffraction angle θ₄. The parallel light beams are then fed to thefocusing lens 50 which, in turn, focuses the incident parallel lightbeams onto the optical disc 60.

The light beams incident on the optical disc 60 are reflected from theoptical disc 60 to be fed back along an optical path reverse to anoriginal optical path extending from the beam splitter 20 to the opticaldisc 60. That is, the light beams reflected by the optical disc 60 arefed to the focusing lens 50 in which they are converted again intoparallel light beams. The parallel light beams enter the collimatinghologram 40 at an incident angle θ₃, so that they are reproduced in theform of light beams with a diffraction angle θ₄. The reproduced lightbeams are reflected by the second mirror 80 and then by the first mirror70. Thereafter, a part of the light beams reflected from the firstmirror 70 enter the beam splitter 20 while the remaining light beamsenter the two-component photodetector 90. The light beams incident onthe beam splitter 20 are then reflected toward the three-componentphotodetector 11.

The three-component photodetector 11 receives the light beams and sensesthem. The light beams detected by the three-component photodetector 11corresponds to the information recorded on the optical disc 60.Accordingly, the information recorded on the optical disc 60 can be readby analyzing the light beams.

The above description has been made, assuming that the optical disc hasnot been subjected to a poor smoothness and a vibration, during theoperation of the optical pick-up system. Where information recorded onoptical discs are read by optical pick-up systems, however, focusingerrors or tracking errors occur frequently, due to a poor smoothness ofdiscs or external vibrations.

The optical pick-up system of the present invention corrects such errorsso that information can be accurately read. An accurate reading ofinformation achieved by the optical pick-up system of the presentinvention is carried out as follows.

First, cases where focusing errors occurred during the operation of theoptical pick-up system will be described, in conjunction with FIGS. 10Ato 10D.

Such focusing errors are generated when the optical disc 60 moves awayfrom or toward the focusing lens 50, as shown in FIG. 10A or FIG. 10B,respectively. Where the optical disc 60 moves away from the focusinglens 50, a reflection point C moves to a point C', as shown in FIG. 10C.On the other hand, where the optical disc 60 moves toward the focusinglens 50, a reflection point C moves to a point C", as shown in FIG. 10D.Assume that S₁ and S₂ are optical signals detected by the photodetectorcomponents D₁ and D₂, respectively and that S₃, S₄ and S₅ are opticalsignals detected by the photodetector components D₃, D₄ and D₅,respectively. When the optical disc 60 moves away from the focusing lens50, as shown in FIG. 10C, light beams are focused in front of thethree-component photodetector 11, as shown in FIG. 14. This is becausethe reflection point C moves to the point C'. In this case, thephotodetector components D₃ to D₅ of the three-component photodetector11 detect the optical signals S₃ to S₅ and thus output photodetectingsignals S₃ ' to S₅ ', respectively. In this case, light beams passingthrough the reflection plate 15 have a size smaller than the diameter lof the opening of reflection plate 15. As a result, the size of lightbeams incident on the two-component photodetector 90 becomes small, sothat the photodetector components D₁ and D₂ of the two-componentphotodetector 90 can not detect the optical signals S₁ and S₂.Therefore, the optical pick-up system recognizes the above case as acase that the optical disc 60 has moved away from the focusing lens 50and thus activates the actuator 12 for shifting the focusing lens 50, sothat it corrects the focusing error, so as to operate normally.

On the other hand, when the optical disc 60 moves toward the focusinglens 50, the reflection point C moves to the point C", as shown in FIG.10D. Accordingly, light beams are focused to the rear of thethree-component photodetector 11, as shown in FIG. 15. In this case, thephotodetector components D₃ to D₅ of the three-component photodetector11 can detect the optical signals S₃ to S₅ and thus outputphotodetecting signals S₃ ' to S₅ ', respectively. In this case, lightbeams passing through the reflection plate 15 have a size larger thanthe diameter l of the opening of reflection plate 15. As a result, thereflection plate 15 reflects a part of the light beams toward thetwo-component photodetector 90, so that the photodetector components D₁and D₂ of the two-component photodetector 90 can detect the opticalsignals S₁ and S₂ and thus output photodetecting signals S₁ ' and S₂ ',respectively. That is, when all the photodetecting signals S₁ ' and S₂ 'from the two-component photodetector 90 and the photodetecting signalsS₃ ' to S₅ ' from the three-component photodetector 11 are not zero (0),the optical pick-up system recognizes this case as a case that theoptical disc 60 has moved toward the focusing lens 50 and thus activatesthe actuator 12 for shifting the focusing lens 50, so that it correctsthe focusing error, so as to operate normally.

When a focusing error has occurred, the movement of the optical disc 60away from the focusing lens 50 results in S₁ '=S₂ '=0. In this case,when S₃ '≠0, S₄ '≠0 and S₅ '≠0, the focusing lens 50 is upwardly shiftedin the y-axis direction, so as to obtain the photodetecting signals S₃ 'and S₅ ' of 0 (S₃ '=S₅ '=0). On the other hand, the movement of theoptical disc 60 toward the focusing lens 50 results in S₁ '≠0 and S₂ ≠0.In this case, when S₃ '≠0, S₄ '≠0 and S₅ '≠0, the focusing lens 50 isdownwardly shifted in -y-axis direction, so as to obtain thephotodetecting signals S₃ ' and S₅ ' of 0 (S₃ '=S₅ '=0). Thus, thefocusing error can be corrected.

Now, corrections for tracking errors which occur during the operation ofthe optical pick-up system will be described, in conjunction with FIGS.11A to 11C, FIG. 12 and FIG. 13.

FIG. 11A shows a condition that the optical pick-up system operatesnormally without any tracking error, whereas FIGS. 11B and 11C showconditions that tracking errors have occurred.

A part of the light beams are reflected by the reflection plate 15 andthen fed to the two-component photodetector 90. In the case of FIG. 11B,the light beams are detected only by the photodetector component D₂ ofthe two-component photodetector 90, as shown in FIG. 12. Accordingly,the photodetector component D₂ outputs a photodetecting signal S₂ ' (S₂'=0), whereas a photodetecting signal S₁ ' of the photodetectorcomponent D₁ becomes zero (S₁ '=0). In this case, the optical pick-upsystem activates the actuator 12 for shifting the focusing lens 50, soas to make S₂ '=0. Thus, the tracking error can be corrected.

In the case of FIG. 11C, the light beams are detected only by thephotodetector component D₁, as shown in FIG. 13. Accordingly, thephotodetector component D₁ outputs a photodetecting signal S₁ ' (S₁'≠0), whereas a photodetecting signal S₂ ' of the photodetectorcomponent D₂ becomes zero (S₂ '=0). In this case, the optical pick-upsystem activates the actuator 12 for shifting the focusing lens 50, soas to make S₁ '=0. Thus, the tracking error can be corrected.

As apparent from the above description, even when an optical disc issubjected to a vibration caused by external noise or has a poorsmoothness, the optical pick-up system according to the presentinvention can achieve an optical pick-up operation with an improvedreliability, by sensing accurately errors caused by the vibration andpoor smoothness and then correcting accurately the errors.

The present invention can also provides a small and light opticalpick-up system, as compared with general optical pick-up systems.Accordingly, it is possible to make the size of the overall systemcompact.

Although the preferred embodiments of the invention have been disclosedfor illustrative purposes, those skilled in the art will appreciate thatvarious modifications, additions and substitutions are possible, withoutdeparting from the scope and spirit of the invention as disclosed in theaccompanying claims.

What is claimed is:
 1. An optical pick-up system for reading informationrecorded on an optical disc, said system comprising:a light source;information reading means for reading said information recorded on saidoptical disc, by using light beams emitted from said light source; errorcorrecting means for detecting an error occurring when said informationreading means reads the information and correcting said error; andsupporting means for providing optical paths for said informationreading means and said error correcting means and supporting both theinformation reading means and the error correcting means; wherein saidinformation reading means comprises: a beam splitter adapted to transmita part of incident light beams therethrough and reflect the remainingpart of said incident light beams; reflecting means adapted to reflectsaid light beams transmitted through said beam splitter, at apredetermined reflection angle, and reflect light beams reflected fromthe optical disc at a predetermined reflection angle; collimating meansfor converting said light beams reflected from said reflecting meansinto parallel light beams; and focusing means adapted to focus saidparallel light beams emerging from said collimating means onto a surfaceof the optical disc, and feed light beams reflected from the opticaldisc to the collimating means; wherein said reflecting means comprises:a first mirror adapted to reflect light beams from said beam splitter ata predetermined angle and light beams carrying said information recordedon said optical disc toward said error correcting means; and a secondmirror adapted to reflect light beams from said first mirror toward saidcollimating means and information-carrying light beams from thecollimating means toward the first mirror; wherein said error correctingmeans comprises: a reflection plate having a aperture surrounded by areflector so as to reflect a part of the information-carrying lightbeams reflected from said information reading means at a predeterminedangle, and to allow the remaining part of said information-carryinglight beams to pass through said aperture; error detecting means,comprising at least two photodetectors, adapted to detect an occurrenceof an error, from said information-carrying light beams, said errordetecting means comprising a first error detector adapted to detect saidlight beams reflected from said reflection plate and a second errordetector adapted to detect said light beams passing through thereflection plate; and actuating means, adapted to adjust saidinformation reading means, based on an error detection result obtainedby said error detecting means; wherein each of said photodetectors isadapted to detect said light beams reflected from said reflection platewhen the error has occurred and not to detect the light beams at anormal operation state of said system.